The Census Bureau conducts a constitutionally mandated decennial census whose figures are used to determine the number of congressional districts to which each state is entitled, in a process called "apportionment". For example, Nebraska had 6 districts until 1933 and only 3 following the 1960 census. The number of districts for Texas increased from 30 to 32 in 2002, and then to 36 in 2012 based on the 2010 census. Other states have lost districts. The 2012 elections were the first to be based on the congressional districts which were defined based on the 2010 census.[3]

Each state is responsible for the redistricting of districts within their state, and several states have one "at-large" division. Redistricting must take place if the number of members changes following a reapportionment, or may take place at any other time if, for example, demographics represented in a district has changed substantially. Districts may sometimes retain the same boundaries while changing their district numbers.

The following is a complete list of the 435 current congressional districts for the House of Representatives, and over 200 obsolete districts, and the six current and one obsolete non-voting delegations.

States with the fewest (only one district "at-large"): Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont, and Wyoming. Alaska, Delaware, and Wyoming are the only states that have never had more than one district. Between 1810 and 1820, Delaware had two Representatives, but they were elected at-large.

The oldest district in the country, it has never changed its shape or size. From 1813 to 1823, Delaware had two representatives—both chosen at-large on a general ticket from the same statewide district.

Until 1820, Maine was part of Massachusetts. After the 1810 census, Massachusetts was allocated 20 districts. Seven Massachusetts districts (then numbered 14 through 20) were credited to Maine soon after it became a state in 1820. See District of Maine.

^ abcdMaine used to be part of Massachusetts. After the 1810 census, Massachusetts was allocated 20 districts. Seven districts (numbers 14 through 20) were credited to Maine soon after it became a state in 1820.